This invention relates to accumulation-mode charge-coupled devices for converting into electrical signals an electromagnetic radiation pattern in a certain wavelength range, particularly but not exclusively for detecting and imaging infra-red radiation.
An accumulation-mode charge-coupled device for converting an infra-red radiation pattern into electrical signals is described in pages 568-570 of Applied Physics Letters, Vol. 25, No. 10, Nov. 15, 1974. The device comprises a semiconductor body having a radiation-sensitive region which is of one conductivity type determined by dopant having an energy level (or levels) sufficiently deep in the semiconductor band gap that substantially all of the atoms of said dopant are un-ionized at the temperature of operation of the device for detecting the infra-red radiation, whereby the said region is substantially free of majority charge carriers in the absence of said radiation and majority charge carriers trapped by said dopant atoms can be released upon excitation by incident infra-red radiation. An ohmic contact to said region is present for supplying to said dopant majority charge carriers to replace charge carriers released by the incident radiation. An electrode system is present on one major side of the region for permitting electric fields to be capacitively generated in the region for accumulating the released majority carriers below the electrode system as charge packets corresponding to the incident radiation pattern and for transporting said charge packets laterally across the region to an output from the region.
The temperature of operation of this known device is 4.degree. K., and phosphorus can be used as the region dopant at this temperature. At room temperature substantially all phosphorus dopant atoms would be ionized and so phosphorus is generally regarded as a shallow-level dopant; however, at 4.degree. K. substantially all of the phosphorus dopant atoms are un-ionized so that at this very low temperature phosphorus is a deep-level dopant. Because substantially all of its dopant atoms are un-ionized, the said region is both substantially free of majority charge-carriers and electrically neutral at the device operating temperature in the absence of the incident radiation. The need to use an operating temperature as low as 4.degree. K. for this known device can be a disadvantage.
The accumulation-mode device described in Applied Physics Letters is an unconventional surface-channel charge-coupled device. It is compared therein with an inversion-mode device arrangement such as occurs in conventional surface-channel charge-coupled devices. In inversion-mode devices charge transfer is of minority carriers, whereas in the accumulation-mode device the charge transfer is of majority carriers (electrons in the phosphorus-doped region). In the accumulation-mode device described the energy bands in the semiconductor region have a linear spatial dependence in the direction of thickness of the region because there is no immobile space charge in this device; thus the electric field is constant throughout the region thickness from the one major side to the opposite major side (neglecting fringing fields between neighboring gates of the electrode system on the one major side). The electric field at an accumulated semiconductor surface is smaller than at an inverted surface because the surface potential in the former case appears across the whole thickness of the region. Thus the charge-packets transferred in the known accumulation device are not so confined against the surface as in an inversion-mode device, and this should reduce the effect of surface trapping states and hence aid transfer efficiency. However, the charge-packets still adjoin the surface during transfer through the region of this known accumulation-mode charge-coupled device so that surface states serving as traps or recombination centers for the charge carriers can adversely affect the charge transfer efficiency, the charge transfer noise, and the speed of operation.